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Question regarding mag mounts

Discussion in 'CB Antennas' started by guitar_199, Jan 11, 2019.

  1. guitar_199

    guitar_199 Member

    Mar 8, 2011
    Likes Received:
    I've never thought much about this....

    If you put a mag mount on a vehicle and tune it... If I move it to another vehicle will it:

    1) still be tuned?
    2) be close enough to use?, or
    3) require complete retuning for the new vehicle?

    Sometimes, I switch the antenna between our two cars... and have never thought about this.

    What say you all?

  2. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

    May 1, 2013
    Likes Received:
    If the metal of the car is approximately the same no tuning would be required.
    I have a Srio 5000 on a magmount and do not require tuning going from Chevy S-10 to the wife's Kia Soul. No noticeable change in SWR.
    The S-10 has it's placement on the cab, and the Kia Soul is in the center of the roof.
  3. Bobcat

    Bobcat Member

    May 24, 2005
    Likes Received:
    That sound s reasonable. Different vehicles here but a very similar case.

  4. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

    May 1, 2013
    Likes Received:
    Just FYI I do check the readings when I change Vehicles.
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

    Oct 23, 2017
    Likes Received:
    I would check it every time. There is a chance that the coax would have a bad spot with all the moving around. I run an SWR meter in line all the time because you never know. I like my radios that much.
    357magnum likes this.
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

    Apr 23, 2018
    Likes Received:
    Ever hear of the Coffee Can method?

    Most mag mounts seem to load up a somewhat workable SWR when placed on the older style (read full metal SEAM and sleeving) coffee cans - I use this method to determine if one on my antennas got "Whacked" by a low tree limb one too many times. High SWR = Better investigate everything.

    Just make sure the Z axis of the can is empty of material before performing the SWR tuning check.

    Don't want the Family getting upset over spilled coffee...

    If the antenna can load up and tune to an acceptable SWR - thunked to the coffee cans lid (or if used - Bottom) that tells me the antenna should tune to the vehicle it will be on. There's enough resonance there at the setup - so when you transfer to the vehicle - if all things were the same - you should show a good or similar SWR reading on the vehicle.

    Too bad that most of todays coffee cans are now cardboard and foil - which does not provide enough area for SWR checks - but if you have some and can use the older ones - then you're blessed...

    Check back on this message - I'm handing a chore at the moment - will continue...

    Ok, had several small fires to attend...

    Ok, the issue around that "Coffee can" method - I was shown this trick by someone wanting a quick way to demonstrate a whip antenna right out of the shrinkwrap the thing came in. So he did up this test "mount" - a simple 8" Mag mount with coax, and a SWR meter and Radio - he did this up inside the store with that coffee can laying bottom side up on the floor - puts the mount centered on the can - threads on the whip and with SWR meter and Radio on - shows how SWR is for that antenna.

    Now, it normally would show a little off but in this case it matched pretty well. At least below 1.5.

    He than takes an MFJ 259 and pretty much gets about the same SWR...with Resistance showing about 35 ~40 ohms or so - this was a Francis 5' Hot-Rod.

    Tells me to take this setup - with the MFJ and go out to your car and try different spots to see what SWR will "turn into" when you mount your antenna in different spots.

    So he came out with me and we "played" around with the system and actually got several spots to show lower SWR than others. He called them "sweet spots" where that antenna likes to be on the car.

    I eventually settled on a roof location for that antenna and made it a permanent mount.

    I can attest that once you locate a spot on your car that a typical Mag mount and antenna - just thrown together - shows a low SWR right off the bat, you may want to put your antenna there. Simply due to the healthy response, or help, the antenna system gets from that location and how well it responds, in low SWR, saves you headaches later on in transferring one antenna to another vehicle.

    You can do this trick to the other vehicles you own to find their best "Sweet spot". Placing that antenna you decide to use right there and leave it alone, and you won't even have to bother retuning. One simple quick check of SWR when you install it - then remember where it goes back to when you take it off for later use. As long as you don't change anything taking it off and putting it back on in the same spot the results will be consistent. Mag mount or permanent - either way - finding the sweet spot of location for the antenna you want to use - makes all the difference when it comes to tuning and getting low SWR and good pickup performance from any antenna.

    But this only works if you help your antenna keep it's original shape, length or form and any other aspects of that antenna - as Right Out Of The Box as possible - you will be in good shape.

    Because of that day back in the 90's - I will always recommend that you use what works for you, but put the antenna where it needs to be for good performance and to obtain that, it requires a good location and a low SWR from the antenna (good resonance) by finding that right location to achieve that\ goal - don't skimp on the importance of the location and antennas willingness to work from that location - you'll regret it if you ignore that simple step.
    #6 Handy Andy, Jan 12, 2019
    Last edited: Jan 13, 2019
    Shadetree Mechanic likes this.
  7. 357magnum

    357magnum Sr. Member

    Feb 25, 2014
    Likes Received:
    I agree w/ Shade . I keep one inline also due to height of my Stryker SRA-10 mounted on my Explorer . Like Andy said that tree limb is always lurking let alone most drive thru's which I now try to avoid . ;)
    Handy Andy, Slowmover and undertaker like this.

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